Hepatitis GB virus C/hepatitis G Virus (collectively will be called HGV) is a recently identified RNA virus which appears to be transmitted by parenteral exposure to contaminated blood and blood products, and may be associated with clinical hepatitis in humans. The objective of this study was to evaluate the prevalence of HGV in HCV infected individuals, and its effect on the severity of HCV infection as determined by the level of alanine aminotransferase (ALT) in the serum. The prevelance of HGV was also investigated in HCV contaminated Immune Globulin Intravenous products (IGIV) manufactured prior to the introduction of viral inactivation processing and in recipients of these lots. Nested primers, specific for the 5' noncoding region of HGV, were designed and used to test serum samples from 100 chronic HCV infected patients, 10 HCV RNA positive IGIV lots and 36 of the recipients of these products. HGV specificity of the PCR products was confirmed by sequencing a number of the amplified products and comparing the results with the published prototype sequence for HGV RNA. HGV RNA was detected in 23 of the 100 (23%) HCV infected patients. The level of ALT was lower in HCV-HGV coinfected patients than those with HCV infection alone. Two of the 10 (20%) IGIV lots tested positive for HGV RNA, however, none of the serum samples from recipients of IGIV contained detectable HGV RNA. These results indicate that HGV infection occurs frequently with HCV infection, probably because of similar modes of transmission and risk factors. However, the severity of HCV infections is not effected by the presence of HGV, as indicated by the level of ALT. This study also shows that the transmission of HGV-RNA from IGIV product to the recipients appear to be less efficient than that seen for HCV.